1. Field of Invention
The present invention relates to a method of fabricating barium titanate powders, and more particularly to a method of fabricating barium titanate powders by normal pressure hydraulic thermal circulation technology.
2. Related Art
Electronic products are being developed in the light and compact trend. High capacity, high operation speed and high reliability are elementary for a newly developed electronic product. High-dielectric constant materials provide the optimal solution to meet the above requirements.
The steatite was used as a dielectric due to its low dielectric constant in the early research years. In 1925, a titanium dioxide ceramic capacitor with a dielectric constant higher than 10 was successfully commercialized. At the time of the Second World War, the dielectric constant of the ceramic capacitor increased to thousands due to the discovery of barium titanate. Barium titanate has drawn interest as a material of electronic appliances due to its high dielectric constant, high piezoelectric constant, low dielectic loss, high impedance, high mechanical-electrical coupling constant and polarizing ability. Therefore, barium titanate can be broadly applied as, for example, MLCC, resistor, resonator, thermistor, tone transducer, semiconducing ceramic and electromagnet absorbant.
A conventional method of fabricating barium titanate powders includes a solid stated method, a precipitation method, a sol-gel method and a high-temperature high-pressure thermal method, which will be described more detailed later.
The solid stated method mixes barium carbonate with titanium dioxide and then performs sintering at high temperature to form barium titanate. However, the barium titanate thus obtained has low purity, and a large and non-uniformly distributed particle diameter. Therefore, the barium titanium obtained by this method is not suitable for application of electronic products.
The precipitation method decreases the distance between atoms to accelerate the solid-phase reaction. Cations required for the reaction are mixed with and intensively stirred in a solution in an amount proportional to reactants. Precipitants may be added to facilitate the deposition. With the reduction of distance between cations from micrometer level to nanometer level, barium titanate powders can be obtained by low-temperature sintering.
There are two methodes of fabricating barium titanate powders by using different precipitants: an oxalate method and a citrate method.
Kim et al. disclose to form barium titanate by using oxalate method in which titanium chloride, barium chloride, and oxalic acid or diethyl oxalate are used as reactants to form BaTiO(C2O4)2 4H2O, J. Mater. Sci., 31(1996), p. 3643–3645 and Park et al. (J. Am. Ceram. Soc., 80(1997), p. 1599–1604. BaTiO(C2O4)2 4H2O is sintered in an oven at high temperature to form cubic barium titanate. Then, a heat treatment is performed to obtain tetragonal barium titanate. This method is simpler and thus has been successfully commercialized. However, all the particles obtained have particle diameters at sub-micrometer level, broad diameter distribution, significant coagulation and poor particle quality. Therefore, it cannot be applied in high-reliability electronic products.
The citrate method is similar to the oxalate method. Ti (OC4H9)4 and citric acid are dissolved in ethylene glycol, and then mixed with BaCO3 in formic acid. The pH value of formic solution is adjusted to form BaTi(C6H6O7)3 6H2O, Tsay and Fang, J. Am. Ceram. Soc., 79(1996), p. 1693–1696. Then, a heat treatment is performed to obtain barium titanate.
Although the product obtained by this method is similar to that by oxalate method, the use of expensive titanium alkoxide as raw material and the use of an organic solvent results in a low economic effect.
A sol-gel method has been proposed to avoid the fact that cations cannot be deposited at the same time due to over saturation in the deposition method. The sol-gel method intensively mixes cation compounds required for reaction. When water is added, the hydrolysis occurs to form solgel. After the solvent is removed, a polymerization occurs to form gel. Then a sintering method is performed after drying. Since cation alcohol compounds quickly reactions with water to occur hydrolysis, and the product M (OR)x-1(OH) and alkoxide or the product itself polymerize with one another, all cations can form crystals to obtain stoichiometric end products.
The method of fabricating high-purity barium titanium using titanium or barium as raw materials includes:
(a) an alkoxide-derived powders method: hydrolysis and dissolution of Ti(OR1)4 and Ba(OR2)2 occur simultaneously to form fine and high-purity barium titanate crystalline powders. The powders are sintered at 500–700° C. in inert gas to remove the residual carbon so as to avoid the fabricating of BaCO3. The reaction mechanism is as follows:Ti(OR1)4+Ba(OR2)2+3H2O→BaTiO3+4R1OH+2R2OHThe product obtained by this method has purity higher than oxalate and citrate. However, the main disadvantage is that the alkoxide is high in cost and sensitive to water.
(b) an alkoxide-derived gel method: this method improves the disadvantage of the above method. Ti (OR)4 and Ba(OR)2 form an amorphous gel at low temperature. The gel is then sintered at high temperature (higher than 400–600° C. to form barium titanate crystals.                (c) sol-precipitation: Ti (OR)4 reacts with acetic acid to form white titanyl acylate precursors. The precursors make polymers solvable in excess of water. Hydroxy titanium acylate is formed to avoid the hydrolysis of titanium alkoxide. The method uses as raw material barium acetate that is much more inexpensive and not so sensitive to moisture. The barium acetate is mixed with titanyl acylate precursors to obtain a barium titanium gel. Then, NaOh solution is added at 85° C. to obtain high-purity cubic barium titanate.        
(d) a sol-gel method: it is similar to the above sol-precipitation method, except that the white titanyl acylate precursors are added into a barium acetate solution, instead of mixed with water, at 25–70° C. to conduct gelation. An amorphous barium titanate gel is obtained. The gel is sintered at 700–1000° C. to form tetragonal barium titanate.
As described above, the sol-gel method uses expensive titanium or barium alkoxide as raw material, which is expensive, and the chemical properties of the raw material are unstable. Furthermore, the method is not easy to control, causing difficulty in scale-up production.
A high-reactivity thermal method is used to keep the cations in atom state. The reactant solution is placed in a close reactor. The temperature and pressure in the reactor increase to facilitate the chemical reaction of the cations. A solid-liquid separation method is used to obtain solids from the product solution. Sintering can be optionally performed to help complete the reaction. The thermal reaction partially dissolves the reactants at high temperature and pressure to become thermal decomposition or oxidation to form crystalline powders. This method has attracted many studies in recent years. Thermal reaction at high temperature and pressure to form barium titanate needs expensive equipment, but lacks high operation safety. Besides, the powders can be made only in batch.
Therefore, there is a need of a method of fabricating high-quality barium titanate powders using inexpensive raw materials.